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Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments

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Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments Syracuse University SURFACE Physics College of Arts and Sciences 11-21-2008 Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments Gianfranco Vidali Department of Physics, Syracuse University, Syracuse, NY Ling Li Syracuse University J. Roser Syracuse University and NASA Ames E. Congiu Syracuse University and Universita di Catania Follow this and additional works at: https://surface.syr.edu/phy Part of the Physics Commons Recommended Citation Vidali, Gianfranco; Li, Ling; Roser, J.; and Congiu, E., "Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments" (2008). Physics. 505. https://surface.syr.edu/phy/505 This Article is brought to you for free and open access by the College of Arts and Sciences at SURFACE. It has been accepted for inclusion in Physics by an authorized administrator of SURFACE. For more information, please contact [email protected]. Analysis of Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments G. Vidalia, V. Pirronellob, L. Lia, J. Rosera,c, G. Manic´ob, E. Congiua,b, H. Mehld, A. Lederhendlerd, H.B. Peretse, J.R. Brucatof and O. Bihamd a. Physics Department, Syracuse University, Syracuse, NY 13244, USA b. Universit´adi Catania, DMFCI, 95125 Catania, Sicily, Italy c. NASA Ames, Mail Stop 245-6, Moffett Field, CA, 94035, USA d. Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel e. Faculty of Physics, Weizmann Institue of Science, Rehovot 76100, Israel and f. INAF-Osservatorio Astronomico di Capodimonte, Napoli, Italy Abstract The study of the formation of molecular hydrogen on low temperature surfaces is of interest both because it enables to explore elementary steps in the heterogeneous catalysis of a simple molecule and due to its applications in astrochemistry. Here we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments, beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations and the energy barriers of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results are used in order to evaluate the formation rate of H2 on dust grains under the actual conditions present in interstellar arXiv:0708.1647v2 [physics.chem-ph] 21 Nov 2008 clouds. It is found that under typical conditions in diffuse interstellar clouds, amorphous silicate grains are efficient catalysts of H2 formation when the grain temperatures are between 9-14K. This temperature window is within the typical range of grain temperatures in diffuse clouds. It is thus concluded that amorphous silicates are good candidates to be efficient catalysts of H2 formation in diffuse clouds. 1 I. INTRODUCTION Few are the studies of the formation of molecular hydrogen on low temperature sur- faces. One of the pioneering experiments was done in the 1970’s by the group of Giacinto Scoles, who measured the scattering, sticking and energy deposition of atomic and molecular hydrogen beams on the surface of bolometers (semiconductor thin films) at liquid helium temperature [1, 2, 3]. It was found that both the sticking coefficient and the hydrogen recombination rate depend on the coverage of H2 on the target surface. It was also shown that the heat released in the formation of molecular hydrogen causes the desorption of hy- drogen molecules that have been pre-adsorbed on the surface. Thus, a molecule just formed is immediately ejected from the surface. These experiments offered a rare view of the inter- action of hydrogen atoms and molecules in the physical adsorption regime and a connection with processes in interstellar space. However, the sample temperature of 3-4 (K) was well below that of interstellar dust grains, the coverage of the sample with atoms/molecules was high and the ice layer not fully characterized. These conditions made it difficult to obtain a quantitative understanding of actual gas-dust grain processes in astrophysical environments. Molecular hydrogen (H2), the most abundant molecule in the Universe, influences the chemical make-up of the Cosmos [4, 5] and is instrumental in the formation of stars by con- tributing to the cooling during the gravitational collapse of molecular clouds. The challenge of explaining the formation of molecular hydrogen in space begins with the realization that the stabilization of the nascent molecule in the bonding of two (neutral) hydrogen (H) atoms involves the forbidden transition to the ground state. Three-body gas-phase interactions are too rare to contribute significantly to H2 formation in cold clouds [4], but may take place in other environments such as interstellar shocks. Under conditions observed in interstellar − − clouds, other gas-phase routes (such as: H+e → H + hν, H + H → H2 + e; or, less + + + + frequently, H+H → H2 , H2 + H → H2 + H ) do not make enough H2 to counterbalance the known destruction rate due to UV photons. [4]. In the 1960’s, Salpeter and collaborators proposed a model in which H2 formation occurs on the surfaces of interstellar dust grains [6, 7, 8, 9]. These grains are formed in the envelopes of massive late-stars and in novae and supernova explosions. They are made of carbonaceous materials and of silicates. Their sizes exhibit a broad power-law like distribution between 1-100 nm [10, 11, 12]. Observations of scattered, absorbed and emitted starlight, and lab- 2 oratory work, show that in the interstellar medium (ISM), silicate grains are amorphous and mostly of composition (FexMg1−x)2SiO4, where 0 <x< 1 [13]. There is, on average, one dust grain per about 1012 hydrogen atoms, and the grains account for about 1% of the mass of interstellar clouds. Kinematic calculations show that in order to produce enough molecular hydrogen to counterbalance the destruction rate, the catalysis on grain surfaces must be efficient. More specifically, the processes of hydrogen sticking, migration and bond formation on the grains must convert at least about ∼ 30% of the adsorbed hydrogen atoms into molecular form [7]. With some exceptions [14, 15], chemical models that look at the chemical evolution of an interstellar cloud, have largely ignored or underplayed the coupling of gas and dust. However, observations, experiments and calculations are pointing to the fact that the formation of key ISM molecules [such as H2, formaldehyde (H2CO), and methanol (CH3OH)] takes place on dust grains, as gas-phase reactions are too slow in these particular cases [16, 17, 18, 19]. As far as the formation of molecular formation is concerned, there is a great need to know the basic mechanisms of reaction (Langmuir-Hinshelwood, Eley-Rideal or hot atom), characteristic energies for various processes (diffusion and desorption) and kinetic parameters of dust-catalyzed reactions so they can be used in models of interstellar chemistry. It is within this framework that in the late 1990’s we began a series of investigations on the formation of molecular hydrogen on analogues of dust grains [20, 21, 22]. These experi- ments, inspired by Scoles’ work, were aimed at combining tools of surface science, chemical physics and low temperature physics in order to recreate the environmental conditions of the interstellar space and overcome some of the limitations of prior experiments, such as: high fluxes of H, too low sample temperatures and not adequately characterized materials (For a review of early experiments, see Ref. [23]). In practice, the experiments have to be done at low background pressure, low sample temperatures and low fluxes of atoms impinging on the samples. The first two requirements are relatively easily achieved. Even taking special care to obtain fluxes of low energy (200-300K) hydrogen atoms, it is not possible to either produce or detect as low fluxes of atoms as appear in the ISM. Thus, carefully designed the- oretical and computational tools need to be used to simulate the actual processes occurring is the ISM using the results of the experiments. The formation of molecular hydrogen on surfaces has been explored at length in the past, but most of the work has been on characterized surfaces of metals and semiconductors, and 3 at much higher surface temperatures and fluxes (or coverages) than in the regime we are interested in. On low temperature surfaces, efficient recombination can occur only if the mobility of hydrogen is high. There are situations when this does not have to be verified, as in the Eley-Rideal and hot atom mechanisms, in which H atoms from the gas phase directly interact with the target hydrogen atoms or move on the surface at superthermal energy. Such mechanisms have been shown to be working in the interaction of H with H-plated metal [24, 25], silicon [26] and graphite [27] surfaces. Although there are certain interstellar environments where these mechanisms enter into play, the diffuse cloud environment - where the coverage of H atoms on a grain at any given time is very low - is not one of them. Thus, we expect that H atoms in our experiments to experience physical adsorption forces and the dominant mechanism of reaction is expected to be the Langmuir-Hinshelwood reaction. In the experiments, the sample is exposed to well collimated beams of hydrogen (H) and deuterium (D) atoms. The production of HD molecules occurring on the surface of a dust grain analogue is measured both during the irradiation with the beams and during a subsequent temperature programmed desorption (TPD) experiment. In order to disentangle the process of diffusion from the one of desorption, additional experiments are carried out in which molecular species are irradiated on the sample and then are induced to desorb.
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